1. Field of the Invention
The present invention relates to a color image recording method and apparatus which form a plurality of color images on a electrostatic latent image formation member such as a light-sensitive member by the use of an electrophotographic recording method and then transfer the plurality of color images to a recording medium together, and relates in particular to a color image recording method and apparatus providing color images of improved quality.
2. Discussion of the Related Art
Conventionally many different color image recording methods adopting an electrophotographic recording method are known. An example of these methods is what is called a superimposing development method, which successively superimposes plural images of different colors on a electrostatic latent image formation member such as a light-sensitive member to form a plurality of images of different colors on it. The plurality of color images is transferred together to a recording medium, resulting in a color image. This method requires only one electrostatic latent image formation member such as a light-sensitive drum, and additionally does not need a transfer drum; therefore the recording apparatus can be miniaturized, and moreover image formation is exceedingly speedy because the plurality of color images can be formed during a single rotation of the electrostatic latent image formation member.
However, the superimposing developing method described above has the problem that the toner image deteriorates to a high degree during the developing process for the second and subsequent colors because the previously developed toner image is damaged as it passes through the developing area again, thus leading to a final color image with severe deterioration. Another problem occurs in the second developing process that part of the toner is rubbed off the toner image formed on the electrostatic latent image formation member in the first developing process and then contaminates the second developer, which causes a reduction of the first toner image density and extreme reduction in the life of the second developer.
To overcome the above-mentioned problems in the superimposing developing method, that is, to develop the second and subsequent latent images without damaging the toner image previously formed on the electrostatic latent image formation member, many techniques have been proposed in relation to the electrostatic latent image formation method or the developing method for the second and subsequent images.
Japanese Patent Application Unexamined Publication No. Sho. 55-36889 (1980) discloses a two-color image reproduction method successively developing two electrostatic latent images formed on the surface of an electrostatic latent image formation member by a pair of magnetic brush developing devices using developers of different colors wherein the second magnetic brush developing device is constructed so that its friction contact force is less than that of the first magnetic brush developing device.
In Japanese Patent Application Unexamined Publication No. Sho. 57-79970 (1982), a two-color developing apparatus for electrophotography having first and second developing devices, forms two electrostatic latent images of opposite polarities on an electrostatic latent image formation member, and develops them with two different two-component toners of different colors. The first and second developing apparatuses have main magnets of different magnetic power, and additionally the main magnets are constructed so that their shape and the distributions of their magnetic flux are different.
The color image recording apparatus disclosed by Japanese Patent Application Unexamined Publication No. Sho. 60-126665 (1985) repeats the process of forming an electrostatic latent image on an electrostatic latent image formation member in a latent image forming section and developing the electrostatic latent image in a developing section a number of times, and then transfers the visible image to a recording medium. Among the plurality of the developing sections described above, at least the second and subsequent developing sections employ a two-component developer made by mixing a non-magnetic toner and a magnetic carrier having a particle diameter of 50 .mu.m or less.
The electrophotographic recording apparatus disclosed by Japanese Patent Application Examined Publication No. Hei. 2-4903 (1990) comprises a means for forming a first latent charge image on a light-sensitive drum member, a first developing means for developing the first latent charge image formed by the means for forming the first latent charge image on the light-sensitive drum member by using a magnetic developer having charged toner of a first color, a means for forming a second latent charge image on the light-sensitive drum member after development by the first developing means, a second developing means for developing the second latent charge image formed by the means for forming the second latent charge image on the light-sensitive drum member by using a magnetic developer having charged toner of a second color, wherein the second developing means comprises a fixed magnet roller whose portion facing the light-sensitive drum member is between two like magnetic poles, a rotating sleeve for carrying developer rotatably installed around the periphery of the fixed magnet roller, a means for applying a DC bias potential having the same polarity as the latent charge image formed on the light-sensitive drum member between the rotating sleeve for carrying developer and the light-sensitive drum member, and a means for applying an AC bias potential to the magnetic developer, which is attached to the surface of the rotating carrying sleeve and carried thereby on the portion facing the light-sensitive drum member, and wherein the spacing between the surface of the magnetic toner attached to the surface of the rotating carrying sleeve and the surface of the light-sensitive drum member facing the magnetic developer is maintained to be 0.05-0.5 mm.
Japanese Patent Application Unexamined Publication No. Hei. 2-77767 discloses a multi-color electrostatic recording apparatus having an electrostatic latent image formation member on which an electrostatic latent image is formed, a developer holding member for holding developer and rotating in a predetermined direction and performing at least two cycles of developing processes by applying a predetermined developing bias voltage to the developer holding member for attaching developer held by the developer holding member to the electrostatic latent image formed on the electrostatic latent image formation member to develop it, wherein 1/2 of the maximum voltage in one waveform cycle of the developing bias voltage applied to the developer holding member is set to be different from the average voltage in one waveform cycle of the developing bias voltage in the second and subsequent developing processes.
However, the conventional art described above has some problems. The inventions disclosed by Japanese Patent Application Unexamined Publications Nos. Sho. 55-36889 (1980), Sho. 57-79970 (1982), and Sho. 60-126665 (1985) are magnetic brush developing methods with a low friction contact force; the developing efficiency or developing capability is lowered in the second developing process, and therefore a sufficient image density cannot be obtained for the second and subsequent colors.
The non-contact developing method disclosed by Japanese Patent Application Examined Publication No. Hei. 2-903 is constructed to maintain the spacing of 0.05-0.5 mm between the surface of the magnetic developer attached to the peripheral surface of the rotating carrying sleeve for the magnetic developer and the surface of the light-sensitive drum member facing the magnetic developer. For this reason, it is difficult to make the pile height of the magnetic brush uniform to a tolerance of 50 .mu.m or less when the spacing is set to be minimum, and besides the low pile density causes a reduction of developing efficiency, thus resulting in insufficient and excessively uneven image density.
Another non-contact developing method in Japanese Patent Application Unexamined Publication No. Hei. 2-77767 (1990) provides an alternating electric field, and by arranging that one half of the peak voltage in a cycle of the bias waveform is different from the average voltage, makes it easier to transfer the developer to the electrostatic latent image formation member, and thus improves development efficiency. In this method, although the developing efficiency is improved, the second toner adheres to the first image area, (referred to as color mixing,) because an electric field strong enough to develop an image is also present in the first image area which has been already developed. Moreover, in the case where the first and second toner are charged with different polarities, an electric field in the direction effective to develop the image with the second toner removes the first toner from the surface of the electrostatic latent image formation member, which aggravates the reduction of the density of the first toner image and deterioration of the second developer because it is contaminated by the first toner.
Further, as one of the other methods related to a non-contact developing method, the developing apparatus disclosed by Japanese Patent Application Unexamined Publication No. Sho. 60-176069 (1985) is also known though it is not necessarily employed for a color image recording method. This invention is characterized in that a horizontal magnetic field is applied to the developer in a developing area, developer is held on a developer holding member without contacting an electrostatic latent image formation member and that an oscillating electric field is used for development. This method requires a strong oscillating electric field to obtain sufficient developing density, and accordingly, if the method is adopted for the superimposing developing method, the problem occurs that first toner image is removed by the effect of the oscillating electric field, thus leading to reduction of density of the first toner image and contamination of the second developer with the first toner.